Structural (XRD) Characterization and an Analysis of H-Bonding Motifs in Some Tetrahydroxidohexaoxidopentaborate(1-) Salts of N-Substituted Guanidinium Cations

The synthesis and characterization of six new substituted guanidium tetrahydroxidohexaoxidopentaborate(1-) salts are reported: [C(NH2)2(NHMe)][B5O6(OH)4]·H2O (1), [C(NH2)2(NH{NH2})][B5O6(OH)4] (2), [C(NH2)2(NMe2)][B5O6(OH)4] (3), [C(NH2)(NMe2)2][B5O6(OH)4] (4), [C(NHMe)(NMe2)2][B5O6(OH)4]·B(OH)3 (5), and [TBDH][B5O6(OH)4] (6) (TBD = 1,5,7-triazabicyclo [4.4.0]dec-5-ene). Compounds 1–6 were prepared as crystalline salts from basic aqueous solution via self-assembly processes from B(OH)3 and the appropriate substituted cation. Compounds 1–6 were characterized by spectroscopic (NMR and IR) and by single-crystal XRD studies. A thermal (TGA) analysis on compounds 1–3 and 6 demonstrated that they thermally decomposed via a multistage process to B2O3 at >650 °C. The low temperature stage (<250 °C) was endothermic and corresponded to a loss of H2O. Reactant stoichiometry, solid-state packing, and H-bonding interactions are all important in assembling these structures. An analysis of H-bonding motifs in known unsubstituted guanidinium salts [C(NH2)3]2[B4O5(OH)4]·2H2O, [C(NH2)3][B5O6(OH)4]·H2O, and [C(NH2)3]3[B9O12(OH)6] and in compounds 1–6 revealed that two important H-bonding R22(8) motifs competed to stabilize the observed structures. The guanidinium cation formed charge-assisted pincer cation–anion H-bonded rings as a major motif in [C(NH2)3]2[B4O5(OH)4]·2H2O and [C(NH2)3]3[B9O12(OH)6], whereas the anion–anion ring motif was dominant in [C(NH2)3][B5O6(OH)4]·H2O and in compounds 1–6. This behaviour was consistent with the stoichiometry of the salt and packing effects also strongly influencing their solid-state structures.

The guanidinium cation is therefore particularly well-suited in stabilizing polyborate anions in the solid state, whereas other nonmetal cations are in general less adaptable. The small size, high symmetry (D 3h ), and high H-bond donor capacity may be contributing factors for this [23,24]. In this manuscript, we investigated the synthesis of new polyborate salts by crystallization from aqueous solutions containing B(OH) 3 and the substituted guanidinium cations. Six new tetrahydroxidohexaoxidopentaborate(1-) salts were obtained (see Figure 1 for schematic structures) and herein we report their synthesis and their solid-state structures as determined by single-crystal XRD studies. Their solid-state H-bond interactions were analysed together with those found in the polyborate salts of the unsubstituted guanidinium cations as a potential means of accounting for this behaviour [25,26].  (1-) found in compounds 1-6 with H-bond acceptor sites labelled as α, β, or γ.  3 in appropriate ratios [19]. It is well-known that B(OH) 3 , and other borate salts, exists in alkaline aqueous solution as equilibrium mixtures of numerous polyborate anions [27,28]. The guanidinium cation that is present templates the crystallization of specific products under the reaction conditions in what can be described as self-assembly processes [29,30].

Results and Discussion
The N-substituted guanidinium starting materials used in this study were all commercially available and were non-carbonate species. We have previously synthesized many nonmetal cation polyborate salts by a room temperature crystallization of aqueous solutions originally primed with an organic free base, or its protonated cation (prepared in situ as its [OH] − salt by a metathesis reaction) and B(OH) 3 [10,11]. Based on this strategy we prepared six new N-substituted guanidinium salts as shown in Scheme 1. Crude yields of these compounds ranged from 71% to near quantitative. The recrystallization of samples from H 2 O gave crystals suitable for single-crystal X-ray diffraction studies.

Scheme 1.
Synthetic procedure for the synthesis of compounds 1-6 (TBD = 1,5,7triazabicyclo [4.4.0]dec-5-ene and the structure of TBDH is illustrated in Figure 1f). Thermal and spectroscopic data (Section 2.2) and elemental analysis data (Sections 3.3-3.9) on the crude products 1-6 were consistent with formulating these materials as N-substituted guanidinium pentaborate salts and these formulations were confirmed by single-crystal XRD studies (Section 2.3). All compounds were colourless and stable in the solid state, insoluble in organic solvents but soluble in H 2 O with decomposition.

Thermal and Spectroscopic Properties
Thermal gravimetric analysis (TGA) data (in air) for nonmetal cation polyborate salts are often reported with a thermal decomposition leading to B 2 O 3 via multistage processes involving the loss of interstitial H 2 O (100-200 • C), the condensation of hydroxy groups bound to boron with the loss of a further two H 2 O molecules (250-400 • C), and then the oxidation of organics (400-700 • C) [10,31,32]. A TGA was undertaken on compounds 1-3 and 6 as representative examples of the new substituted guanidinium pentaborate(1-) salts. Compounds 2, 3, and 6 did not have interstitial H 2 O and their TGA curves showed the expected loss of two H 2 O molecules between 240 and 320 • C. Compound 1 followed this expected behaviour but there was no clear distinction between the two lower temperature processes that occurred (100-275 • C) with the loss of three H 2 O molecules. The glassy solids that remained after heating to 700 • C for compounds 1-3 and 6 had residual masses consistent with 2.5 B 2 O 3 . 1 H and 13 C NMR were obtained for compounds 1 and 3-6 dissolved in D 2 O and these all gave spectra consistent with the appropriate guanidinium cations being present, e.g., the 1 H spectrum of compound 4 gave a signal for the N-Me groups as a singlet at 2.85 ppm with exchangeable H atoms with HOD (4.7 ppm). The 13 C{ 1 H} spectrum of compound 4 gave two signals at 36.98 and 160.2 ppm for the CH 3 and CN 3 carbons, respectively, with the downfield signal being weak. The presence of boron in these compounds was confirmed by 11 B NMR (in D 2 O) for all compounds. All 11 B spectra all showed three "signature" signals [31,33], arising through the decomposition of a pentaborate, with relative intensities appropriate for a sample that had attained polyborate/aqueous equilibrium [27]. Thus, compound 4 gave three signals at 1.0, 12.8, and 19.1 ppm with approximate relative intensities of 5%, 10%, and 85%, respectively. These signals have been previously assigned, moving downfield, to [B 5 O 6 (OH) 4 [27]. Three signals were observed as the sample was relatively concentrated rather than one signal at +16.1, which would have been expected at infinite dilution [33].
All samples were characterized by FTIR spectroscopy. Strong and potentially diagnostic absorptions were to be expected for the cation ν(NH) at ca. 3400 cm −1 and ν(CN) at ca. 1650 cm −1 [34] and the polyborate anion ν(O-H) at 3500 cm −1 and the ν(B-O) stretches grouped between 1450 and 740 cm −1 [35]. In particular, a strong adsorption at ca. 925 cm −1 in the (B trig -O) sym stretching region has previously been described as diagnostic for the [B 5 O 6 (OH) 4 ] − anion [36]. This strong absorption, amongst other strong B-O stretches, together with the strong adsorptions associated with the cation (1671-1625 cm −1 ), was present in all samples.

Single-Crystal XRD Studies
Compounds 1-6 were characterized by single-crystal XRD studies. All the compounds contained the expected insular N-substituted guanidinium(1+) cations and insular tetrahydroxidohexaoxidopentaborate(1-) anions within the asymmetric unit. Additionally, compound 1 had one H 2 O of crystallization per cation/anion and compound 5 was cocrystallized with a molecule of B(OH) 3 per cation/anion. Crystallographically, compounds 1-5 were free of disorder, whilst compound 6 was modulated, but was only refined using the subcell with all atoms disordered over two positions of equal population. Brief crystallographic details for each compound are given in the experimental section, and atomic numbering schemes for compounds 1-6 are shown in Figure 2. The full crystallographic information is available as Supplementary Materials. Generally, the gross structures, bond angles, and bond distances within the guanidinium [17,37] and pentaborate [31][32][33]36] units were within the expected ranges for these ions and need no further comment. The N-substituted guanidinium cations had a variable number of potential H-bond donor sites (one to seven in this study) that were dependent on the extent of the substitution. Each pentaborate(1-) unit had four potential H-bonds donor sites and ten potential H-bond acceptor sites in locations described as α, β, or γ, as defined elsewhere [10,32], and they are illustrated in Figure 1. Since solid-state H-bonding is likely to be important in stabilizing these structures [23][24][25][26], the H-bond interactions in compounds 1-6 were analysed and are described in detail using an Etter graph set terminology [38]. Compounds 2, 3, 4, and 6 had the extended anion-anion H-bonded lattices that are found in many nonmetal cation pentaborate salts [10], with the cations filling the "voids" and "channels" and forming additional H-bonds to the anions. In these structures, each pentaborate(1-) anion donates four H-bond to four neighbouring pentaborate(1-) anions. The extended lattice structures are 3-D because the central B atom in each pentaborate (1-) is tetrahedral with its two associated boroxole rings perpendicular to each other (D 2d for heavy atoms).
Compound 5 had B(OH) 3 cocrystallized with the pentaborate salt and consequentially had a unique anionic pentaborate/boric acid lattice. However, the R 2 2 (8) building motifs seen in most polyborate structures were also readily identified here. Thus, one "plane" was comprised of chains of pentaborates each linked via two reciprocal-α rings. These reciprocalα interactions originated from O7-H7 and O8-H8 with O3 and O1 acceptors, respectively. The other two H-bond interactions originating from each pentaborate (O9-H9 and O10-H10) are shown in Figure 4. The interactions shown here were approximately perpendicular to those just described an originating from O7-H7 and O-H8. The "horizontal" chains in Figure 4 were comprised of alternating pentaborate(1-)/boric acid moieties with the H-atom positions of the B(OH) 3 acid arranged to maximise R 2 2 (8) interactions. Thus, O10-H10 on the pentaborate and O12-H12 on the B(OH) 3 were involved in a standard borate-borate interaction and O11-H11 and O13-H13 were involved in a "pincer" double H-bond to the pentaborate at O4 (α) and O9 (β) sites. Each B(OH) 3 formed three donor H-bonds to two α sites and one β site of neighbouring pentaborates. The fourth and final H-bond originating from each pentaborate is also shown in Figure 4. Here, O9-H9 formed an Hbond to a β-acceptor site (O8) of its neighbour, cross-linking the pentaborate(1-)/boric acid chains into the "plane" shown in Figure 4 by the formation of C(8) chains. The sterically demanding [C(NHMe)(NMe 2 ) 2 ] + cation in compound 5 had only one amino group (N1-H1) available for H-bonding and this was utilized in the H-bonding to O11 of the B(OH) 3 ( Figure 4). The B(OH) 3 in this structure served to expand the anionic pentaborate lattice to enable the sterically demanding cation more space. This behaviour has been observed before in several polyborate [36,[41][42][43]

Discussion on H-Bonding in Guanidinium Polyborates
The anions in polyborate salts, [B a O b (OH) c ] n− , are invariably rich in H-bond donor sites (there are c donor BOH groups) and oxygen-atom H-bond acceptor sites with acceptor sites (b + c) outnumbering the donor sites and enabling the opportunity for H-bond donor cations to contribute to the H-bonding networks. R 2 2 (8) are important stabilizing interactions [10,33] and the maximum number of borate-borate R 2 2 (8) rings available to a polyborate anion is limited to the lower value of b or c, since bridging oxygen atoms are acceptors in these R 2 2 (8) interactions. In actual structures, packing/steric effects may reduce further this maximum number.
As noted in the introduction, the G cation was found in three polyborate structures. These "oxoanions" are ideally set up to accept the pincer R 2 2 (8) rings, and we examined, focussing on the polyborate anion, the extent of such interactions as they are in direct competition (see below) with borate-borate R 2 2 (8) rings in stabilizing these structures. We also examined how N-substitution affected this R 2 2 (8) balance and how this affected the observed structures.
When considering solid-state H-bonding interactions the generalised guanidinium (either N-substituted or unsubstituted) polyborate salt can be represented as (xG) n [B a O b (OH) c ] where x represents the maximum number of potential donor pincer R 2 2 (8) motifs available to the cation. Each unsubstituted G cation has the potential to form a maximum of three donor pincer R 2 2 (8) motifs (i.e., x = 3), whilst substituted G cations will have fewer opportunities (x = 0, 1, or 2) with the value of x dependent on the extent and position of the substitution. Within guanidinium polyborate salts, the maximum number of R 2 2 (8) acceptor interactions (both pincer and normal) is b, since bridging oxygen atoms generally can only be used once and are required for both types of these R 2 2 (8) interactions. Usually, borate-borate interactions are approximately coplanar with boroxole rings. Packing/steric effects may reduce the maximum number for both types of interactions.
This analysis focused on an analysis of two ratios. Firstly, a value of >1 for (xGn + c)/b would indicate borate-borate and pincer R 2 2 (8) motifs were in direct competition for the available borate acceptor sites. Secondly, potential and actual (i.e., observed) (xGn)/c ratios for each compound were also of interest since a difference in these values may indicate a structural preference for one or the other of these H-bonding motifs.
The examination of the structure of G 2 [B 4 O 5 (OH) 4 ]·2H 2 O [17] revealed two independent G cations. Two molecules of H 2 O within this formula unit also affected the structure but did not affect this analysis. All five bridging oxygen atoms of the tetraborate(2-) anion were involved in R 2 2 (8) rings with four cation-borate pincer interactions and only one borate-borate interaction. Thus, within this structure, 80% of these rings involved G cations and the observed (xGn)/c ratio was 4.0. This value was much higher than would be expected (1.5) based on the number of potential donor sites. The higher-than-expected observed (xGn)/c ratio would indicate that the G cations were either able to replace the strong borate-borate interactions with stronger pincer motifs or that additional factors were also important. Such factors may include the compound's stoichiometry and that the three-fold symmetry of G cations (D 3d ) were a good packing match for the tetraborate(2-) anion (C 2v ). H-bonding in tetraborate(2-) structures not containing the G cation have been recently reviewed [50] and their structural architectures are, as expected, dominated by borate-borate interactions with the majority of structures having at least two R 2 2 (8) rings. Within the structure of G 3 [B 9 O 12 (OH) 6 ] [19], all twelve bridging oxygen acceptor sites were involved in eleven R 2 2 (8) H-bonded rings with seven pincer charge-assisted interactions involving the three G cations and four borate-borate interactions. Interestingly, the four bridging oxygen atoms joined to the central boron were involved, solely amongst themselves, in three pincer rings; the two oxygens that bridged two four-coordinate borons were both involved in two pincer rings. These rings were not coplanar with boroxole rings. In this salt, 64% of the R 2 2 (8) rings involved the G cations and 36% were borate-borate. This observed ratio of 1.78 was higher than that expected (1.5) based on total potential donor sites and would again indicate that strong cation-anion pincer interactions were able to replace the strong borate-borate interactions, but again, stoichiometry and packing considerations were also important.
The H-bonded structure of G[B 5 O 6 (OH) 4 ]·H 2 O [18] utilizes only four of the six available acceptor bridging oxygen atoms in R 2 2 (8) interactions. Here, and in contrast to the tetraborate(2-) and nonaborate(3-) structures, 75% of the H-bond ring interactions were borate-borate R 2 2 (8) rings with the charge-assisted pincer rings only accounting for 25%. The observed (xGn)/c ratio of 0.33 was lower than would be expected (0.75) from the number of potential donor sites and confirmed that the borate-borate interactions strongly influenced that structure and that the templating influence of G was minimal. These structures were strongly stabilized by these anion-anion interactions with any additional cation-anion interactions helping to further stabilize the anionic lattice [10]. Many non-metal cation pentaborate(1-) salts show three such borate-borate interactions [10]. This was also the likely situation in this salt since the three-fold symmetry of the cation did not pack well with the D 2d symmetry of the insular pentaborate(1-) anion and its perpendicularly H-bonded giant lattice.
Adding substituents to the G cations affects their structure directing potential in three ways: it reduces their ability to form H-bonds, increases their steric bulk, and disrupts their three-fold symmetry. The first factor also correspondingly lowers x, reduces the (xGn)/c ratio, and hence encourages more borate-borate H-bonds. The structures of the N-substituted guanidinium cation pentaborate(1-) salts 1-6 were described in detail in Section 2.2, but further general comments based on the analysis above would now be appropriate. The numbers of potential pincer H-bond donor sites (x) available in compounds 1-6 were 2, 2, 1, 0, 0, and 1, and the (xGn)/c ratios were lower than that for G[B 5 O 6 (OH) 4 ] (0.75) at 0.5, 0.5, 0.25, 0, 0, and 0.25, respectively. This analysis would indicate borateborate interactions should dominate the H-bond interactions in compounds 1-6, with additional charge-assisted pincer H-bonds forming wherever possible. Unsurprisingly, three or four borate-borate R 2 2 (8) rings were observed in compounds 1-6 and these interactions were clearly important in stabilizing their solid-state structures. Pincer interactions were unavailable for compounds 4 and 5 and the amino-H atoms formed simple H-bonds to borate/boric acid acceptor sites. Compounds 3 and 6 both had the potential to form one pincer bond to stabilize the lattice and each had one such pincer bond. Compound 2 had the potential to form two pincer bonds, and both were formed. Compound 1 had potentially two pincer bonds to the borate but here, only one was observed. However, this was a direct consequence of the cocrystallized H 2 O since the H 2 O was part of an alternative R 2 1 (6) pincer ring motif. Compound 5 had a molecule of B(OH) 3 cocrystallized and no opportunity to form a pincer ring. As noted in Section 2.2, the [C(NHMe)(NMe 2 ) 2 ] + in compound 5 was relatively bulky and the B(OH) 3 served to extend the anionic lattice, by positioning itself between two pentaborate(1-) anions. It is interesting to note that the H-atom positions in B(OH) 3 were asymmetric (see Figure 4) and therefore allowed it to partake in an R 2 2 (8) donor pincer interaction to one pentaborate(1-) and a standard R 2 2 (8) borate interaction to the other.

General
Reagents were all obtained commercially. FTIR spectra were obtained as KBr pellets on a Perkin-Elmer 100FTIR spectrometer (Perkin-Elmer, Seer Green, UK). 1 H, 11 B, and 13 C{ 1 H} NMR spectra were obtained on a Bruker Ultrashield Plus 400 spectrometer (Bruker, Coventry, UK) on samples dissolved in D 2 O at 400, 128, and 100 MHz, respectively. Chemical shifts are in ppm with positive values to the high frequency (downfield) of TMS ( 1 H, 13 C) or BF 3 . OEt 2 ( 11 B). TGA and DSC were performed on an SDT Q600 V4 instrument (TA Instruments, New Castle, DE, USA) using Al 2 O 3 crucibles with a temperature ramp-rate of 10 • C per minute (20-700 • C in air). X-ray crystallography was performed at the EP-SRC national crystallography service centre at Southampton University. Drawings in this manuscript have been generated using Mercury software (CCDC, Cambridge, UK). CHN analyses were obtained from OEA Laboratories (Callingham, Cornwall, UK).

Preparation of [C(NHMe)(NMe 2 ) 2 ][B 5 O 6 (OH) 4 ]·B(OH) 3 (5)
[C(NHMe)(NMe 2 ) 2 ]I (1.0 g, 4.0 mmol) was dissolved in H 2 O (25 mL) along with a DOWEX 550A (OH) − ion exchange resin (18 g) and stirred for 18 h. The resin was removed by filtration and the filtrate was added to B(OH) 3 (1.3 g, 21 mmol). The solution was stirred for 2 h and evaporated under reduced pressure to yield 1.4 g of a white powder as a crude product (97%). A 0.3 g sample of the product was redissolved in H 2 O (15 mL) and white crystals suitable for X-ray diffraction studies were obtained after a few days.  (20 mL) and to this was added B(OH) 3 (2.2 g, 36 mmol). The resulting solution was stirred under gentle heating to fully dissolve the B(OH) 3 and left for 3 h. The solution was then evaporated under reduced pressure to yield a crude white product (2.4 g, 6.7 mmol, 93%).